I. Technical Field
The present invention pertains to wireless telecommunications, and particularly to determining whether to enhance diversity in an Orthogonal Frequency-Division Multiplexing (OFDM) system.
II. Related Art and other Considerations
Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath-channels (i.e., the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information.
Diversity techniques are used for reducing the errors in the transfer of a single data stream. Diversity gives an increase in the robustness of the signal path. This means there will be an increase in the maximum data rate at any given distance.
Multi-carrier based radio access schemes such as Orthogonal Frequency-Division Multiplexing (OFDM), Multi-Access OFDM, and Discrete Fourier Transform (DFT)-spread OFDM have been treated as the most promising candidates for many standards due to their capabilities of combating multi-path propagation and supporting frequency-domain multi-user diversity, like 3GPP-LTE, WLAN(802.11n) and WiMAX (802.16). For both single-user frequency diversity mode and multi-user diversity mode, the achievable gain depends on frequency selectivity over the whole spectrum. The frequency selectivity is determined by, e.g., the practical channel condition. Generally, a small delay spread ends to a very flat channel in frequency domain, where the frequency-domain multi-user diversity gain could be very limited. An extreme example is the line of sight (LoS) channel.
To solve the problem of limited gain, a method called cyclic delay diversity (CDD) has been proposed with multiple antennas at transmit side. Cyclic Delay Diversity (CDD) is a technique which introduces spatial diversity to an Orthogonal Frequency Division Multiplexing (OFDM) based transmission scheme that itself may have no built-in diversity. CCD is described in the following non-exhaustive list of documents (all of which are incorporated herein by reference in their entirety):    A. Lodhi, F. Said, M. Dohler, and A. H. Aghvami, “Performance comparison of space-time block coded and cyclic delay diversity MC-CDMA systems,” in IEEE Wireless Communication Magazine, pp. 38-45, April, 2005    G. Bauch, J. S. Malik, “Parameter optimization, interleaving and multiple access in OFDM with cyclic delay diversity,” In proc. VTC 2004, pp. 505-509, 2004    Samsung, R1-051046, further details on adaptive cyclic delay diversity scheme, 3GPP TSG RAN WG1 meeting 42 bis, San Diego, USA, 10-14 October, 2005.    Samsung, R1-051047, System performance of adaptive cyclic delay diversity scheme, 3GPP TSG RAN WG1 meeting 42 bis, San Diego, USA, 10-14 October, 2005    Peter Larsson, “Cyclic delay diversity for mitigating intersymbol interference in OFDM systems”, U.S. Pat. No. 6,842,487, prio.-date Sep. 22, 2000    R1-063345, “CDD-based Precoding for E-UTRA downlink MIMO”, RAN1 #47, LGE, Samsung, NTT-Docomo.
CDD-based precoding can be defined by combining a linearly increasing phase-shift diagonal matrix and a unitary precoding matrix as shown by Expression (1). For instance, the CDD-based precoding matrix for the number of transmit antennas Nt with spatial multiplexing rate can be defined by combining a phase-shift diagonal matrix and a precoding matrix. In Expression (1), k and θi,i−1, . . . Nt−1 denote subcarrier index and phase angles according to the delay samples respectively.
                              [                                                    1                                                              0                  ⁢                                                                          ⁢                  …                                                            0                                                                    0                                                              ⅇ                                      j                    ⁢                                                                                  ⁢                                          θ                      1                                        ⁢                    k                                                                              0                                                                    ⋮                                            ⋱                                            ⋮                                                                    0                                                              0                  ⁢                                                                          ⁢                  …                  ⁢                                                                          ⁢                                      ⅇ                                          j                      ⁢                                                                                          ⁢                                              θ                                                  N                          -                          1                                                                    ⁢                      k                                                                                                                                                                                  ]                ⁡                  [                                                                      R                  ⁢                                                                          ⁢                  columns                  ⁢                                                                          ⁢                  of                                                                                                      size                  ⁢                                                                          ⁢                  Nt                                                                                                      Unitary                  ⁢                                                                          ⁢                  matrix                                                              ]                                    Expression        ⁢                                  ⁢                  (          1          )                    
The signals transmitted from different antennas are copies of one time-domain OFDM symbol, each copy with different amount of cyclical shifts. For OFDM system, by doing so, an artificial multipath environment is generated to provide or enlarge the frequency selectivity. Apparently, the system performance depends on the cyclic delay value. In G. Bauch, J. S. Malik, “Parameter optimization, interleaving and multiple access in OFDM with cyclic delay diversity,” In proc. VTC 2004, pp. 505-509, 200, a methodology to determinate cyclic delay value is presented without the consideration of sub-carrier allocation. In others of the above-listed documents, several methods are proposed together with sub-carrier allocation which suggests that cyclic delay should be used for the frequency-domain multi-user diversity mode whereas one large valued set of cyclic delay should be used for the single-user frequency-domain diversity mode. That is, two types of delay samples such as a large delay sample and a small delay sample are used for different cases: the CDD-based precoding with the large delay sample in the transmit antennas is used to obtain transmit diversity gain, and multi-user frequency domain scheduling with small delay sample in the transmit antennas is used to obtain multi-user diversity.
There is no single multi-antenna solution that works well for all the scenarios with different channel conditions, antenna configurations, bandwidths, terminal capabilities and user mobility. Consequently, to ensure good system spectrum efficiency, the adaptive multi-antenna technology has received more and more attention recently. As an example, spatial-domain multiplexing with precoding and dynamic rank adaptation is the most promising solution. Spatial domain multiplexing supports multi-stream transmission among multiple antenna elements, which works very well at the high-rank channels. However, for the low-rank channels, e.g., less-scattering channel or with small transmit antenna separation, multi-stream transmission ends to strong inter-stream interference, thus the single-stream with beamforming is preferred.
The (fractional) frequency reuse is a well known technology. See, for instance, U.S. Pat. No. 6,088,416, incorporated herein by reference. Frequency reuse has the ability to use the same frequencies repeatedly across a cellular system, since each cell is designed to use radio frequencies only within its boundaries, the same frequencies can be reused in other cells not far away with little potential for interference. The reuse of frequencies is what enables a cellular system to handle a huge number of calls with a limited number of channels. On the other hand, The Inter-cell Interference Coordination (ICIC) technology has the task to manage radio resources (notably the radio resource blocks) such that inter-cell interference is kept under control. See, e.g., 3GPP TS 36.300, “Technical Specification Group Radio Access Network Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”, 2007-02, incorporated herein by reference. As used herein, a resource block is a number (M) of consecutive sub-carriers for a number (N) of consecutive OFDM symbols.
For an OFDM system, introducing CDD in precoding can introduce a linear phase shift to the frequency channels, which can help to obtain frequency scheduling gain in the flat channel scenario. See, e.g., Samsung, R1-051047, System performance of adaptive cyclic delay diversity scheme, 3GPP TSG RAN WG1 meeting 42 bis, San Diego, USA, 10-14 Oct. 2005. FIG. 1 shows Mean user throughput PFTF for per stream rate control (PARC) and selective per stream rate control (S-PARC) with and without CDD preceding in single cell with flat channel. Thus, FIG. 1 shows that, in the single-cell with flat channel scenario, the CDD can improve system performances for the frequency-domain scheduler, e.g., PFTF (proportional fair in both time and frequency domain), since the CDD can get more frequency channel variation, and due to the fact that there is only white noise, the fading variation of the frequency channel has an effect of the SINR variation in the frequency domain. FIG. 1 shows mean user throughput PFTF for per stream rate control (PARC) and selective per stream rate control (S-PARC) with and without CDD preceding in single cell with flat channel.
However, whether a CDD-based linear phase shift scheme can obtain more multi-user gains (e.g., by a frequency domain scheduler) over the system without CDD depends on whether it can obtain more frequency domain SINR variations. Not only the channel models, but also interference distribution and whether rank adaptation is used will impact its performance. FIG. 2 illustrates mean user throughput PFTF for PARC and S-PARC with and without CDD preceding in a multi cell with suburban SCM channel. FIG. 2 thus shows that CDD does not provide any interesting gains in a multi-cell scenario with frequency reuse equal to one. See, e.g., 3GPP TR 25.814, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Layer Aspects for Evolved UTRA (Release 7)”. Different UEs have different interference distribution, with frequency domain scheduler, different UEs are allocated to different resource blocks, the variation of interference in resource blocks have already introduced different SINR distributions to the frequency domain resource blocks. Thus, the CDD-based linear phase shift scheme could not introduce more multiuser diversity gain in the multi-cell scenario with suburban SCM channel
What is needed, therefore, and an object of the present invention, are one or more of apparatus, methods, and techniques for selectively implementing CDD based on interference distribution and environment.